1. Field of the Invention
The invention relates to phosphor-converting white light emitting devices, and more particularly to a light emitting device uniformly and conformally covered by a luminescent material layer.
2. Description of the Related Art
Phosphor-converting white light-emitting diodes (LEDs) adapt a separated phosphor coating layer from a blue LED chip to produce light emitting devices which emit white light, i.e., light emitting devices in which a blue LED is disposed in a reflective cup and is surrounded by material including phosphors. A portion of the blue light emitted by the LED is absorbed by the phosphors, which in response emit red and green light. The combination of the unabsorbed blue light emitted by the LED and the red and the green light emitted by the phosphors can appear white to the human eye.
FIG. 1 is a schematic view of a conventional phosphor-converting white light-emitting diode. Referring to FIG. 1, a phosphor-converting white LED 10 includes a blue light emitting diode chip 30 oriented in a flip chip arrangement on a substrate 20. The light emitting diode chip 30 is electrically connected to the substrate 20 through ball grid bonds 35. A phosphor layer 40 is formed overlying the package of the light emitting diode chip 30 on the substrate 20. The package is attached to a reflector cup 15 by an adhesive 18. Conventional methods adapt applying a phosphor particle containing resin on the blue LED. The phosphor particle is then uniformly deposited onto the interface of the blue LED. Uniform deposition of the phosphor particle, however, is very difficult to be implemented. Thus, non-uniform deposited phosphor surrounding LEDs frequently emit white light in a central cone surrounded by annular rings of yellow and blue light, rather than a uniform white spatial profile.
U.S. Pat. No. 6,650,044, the entirety of which is hereby incorporated herein by reference, discloses a light emitting device with a stenciled phosphor layer on an LED chip which is oriented in a flip chip arrangement. FIG. 2A is a schematic cross section of a conventional light emitting diode after stenciling with a phosphor layer. In FIG. 2A, a light emitting device includes a first semiconductor layer 68 of a first conductivity type and a second semiconductor layer 64 of a second conductivity type. The semiconductor layers 68 and 64 are electrically coupled to an active region 66. The active region 66 is, for example, a p-n junction associated with the interface of layers 68 and 64. An optional transparent cap 70 is disposed on the semiconductor layer 68. The contacts 56 and 62 are electrically coupled to the semiconductor layers 68 and 64, respectively. The interconnects 59 and 57 electrically couple the contacts 56 and 62 to the substrate contacts 55 and 53, respectively. The light emitting device is attached onto a substrate 51 in a flip chip arrangement. A transparent layer 75 is formed overlying the light emitting device and a phosphor layer 80 is stenciled on the transparent layer 75.
Conventional flip chip packages, however, are typically costly and less reliable than conventional wire-bond packages. Particularly, when a flip chip LED 60 is misaligned or tilted, a bulge 85 and a trail 86 may be formed during the stenciling process or as the stencil is removed, as shown in FIG. 2B. Therefore, additional care must be taken to avoid generation of bulges or trails that would lead to an uneven attachment of the flip chip LED on a substrate. Uneven stenciled phosphor layers can results in non-uniform color correlated temperature (CCT) and deteriorate reliability.
Accordingly, a light emitting device, with uniform deposited phosphor layers and without results of bonded connection problems is desired.